Default to zsh distro-wide (live user + useradd default)

BOS shipped zsh + a p10k skel .zshrc and Calamares' userShell was already
/bin/zsh, but two paths still defaulted to bash:
  - /etc/default/useradd had SHELL=/usr/bin/bash, so any plain `useradd` (and
    anything not going through Calamares) created bash users.
  - bos-live-setup created the live ISO user with -s /bin/bash, so the live
    session ran bash instead of the BOS zsh setup.

Ship /etc/default/useradd with SHELL=/usr/bin/zsh and create liveuser with zsh
so the whole distro — live and installed — defaults to zsh.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

iso/airootfs/usr/local/bin/bos-live-setup

@@ -11,7 +11,7 @@ set -e
 # (breadd + breadbar + breadbox + keybinds) — proper live-media functionality,
 # not an installer kiosk.
 if ! id liveuser &>/dev/null; then
-    useradd -m -s /bin/bash liveuser
+    useradd -m -s /usr/bin/zsh liveuser
     for g in wheel video input audio storage power; do
         getent group "$g" >/dev/null 2>&1 && gpasswd -a liveuser "$g" >/dev/null || true
     done